This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. During the replication process, HIV-1 viruses selectively package two copies of their full-length unspliced RNA genomes from a cellular pool that contains a substantial excess of cellular and spliced viral RNAs. Understanding the genome packaging mechanism will be essential for the development of new anti-AIDS strategies. High-resolution structural studies of the viral RNA genome fragment can provide valuable information in unraveling the mechanism. Recent in vivo studies have indicated that the full length HIV-1 5[unreadable]-untranslated region (UTR) and upstream coding regions of the gag gene are responsible for efficient genome packaging. However, the large molecular size (~3000 kDa) has posed extreme difficulties for NMR characterizations due to the resonance overlap and peak broadening. To reduce the complexity of spectra, we employed segmental isotope labeled RNA samples to investigate the intact 352 nucleotide HIV-1 5[unreadable]-UTR structure. It will be essential if cryo-EM can provide some structural constraints for this large RNA. In addition, we are expecting that cryo-EM can "visualize" the 5'-UTR conformation change upon dimerization.